1. Field
The disclosed embodiments relate to a system whose purpose is to limit the vertical load applied to the forward landing gear of aircraft, as well as the oscillations of the said aircraft when braking is performed on the ground, including, in particular, low-speed emergency braking.
The disclosed embodiments are applicable to the field of aeronautics, and, more specifically, is intended for heavy aircraft equipped with a set of forward landing gear.
2. Brief Description of Related Developments
Generally speaking, an aircraft is equipped with multiple sets of landing gear, whose essential functions consist primarily of allowing the aircraft to maneuver on the ground, which functions include taxiing, turning, and braking, and, secondarily, of absorbing the vertical kinetic energy of the aircraft upon landing. Each landing gear includes one or more wheels, and is equipped with a shock absorber that ensures comfort during taxiing and dampens the impact upon landing.
Modern heavy aircraft include two separate types of landing gear: the main gear, on the one hand, which are affixed under the wing and/or under the fuselage, and the forward landing gear (which in rare cases may consist of two juxtaposed sets of forward landing gear), which are affixed under the fuselage forward of the main landing gear. Because the aircraft needs at least three points of support on the ground in order to be stable, the most simple configuration (known as the “tricycle” configuration) consists of two sets of main landing gear (left and right) mounted symmetrically under the wings or the fuselage, and one set of forward landing gear.
The purpose of the main landing gear is to withstand the majority of the loads transmitted between the ground and the aircraft, during taxiing and also during landing. The main landing gear also provide the braking function, with each of their wheels (or sometimes only a portion of the wheels) being equipped with a brake.
Meanwhile, the forward landing gear, whose primary role is to provide the additional point of support that is essential to the stability of the aircraft on the ground, absorbs much less of the load than the main landing gear. Generally speaking, its principal function is to steer the aircraft on the ground; for this purpose, the wheel or wheels of the forward landing gear are steerable.
Thanks to the configuration of modern aircraft, as described hereinabove, the forward landing gear is subjected to much weaker loads than the main landing gear for the majority of the maneuvers on the ground, including low-speed or moderate-speed taxiing, turns, and landing.
However, this is not the case when the aircraft performs violent braking. As a result of the effect of the sudden and continuous application of a braking effort on the main landing gear, the aircraft performs a rapid diving motion around its pitch axis, which motion compresses the shock absorber of the forward landing gear and applies a high vertical load to this landing gear, along with major loads on a structure of the aircraft (for example, a sharp load and a flexing moment of the forward fuselage).
Furthermore, when the compression of the shock absorber of the forward landing gear has reached its maximum point, the pressure applied to the shock absorber is relieved, and the shock absorber returns part of the energy that was stored during the compression phase. Because the braking load on the main landing gear continues to be applied during this period, the resulting diving effect halts the release of energy by the forward landing here, and initiates another compression phase. The phenomenon is repeated, such that a series of compressions and releases of the forward landing gear occurs, which is reflected in a pitch oscillation by the aircraft (at the same frequency of the aircraft resting on its landing gear, i.e., on the order of 1 Hz for the heaviest aircraft), with the loads on the forward landing gear and in the structure of the aircraft oscillating at the same frequency. Is oscillations may persist for a fairly long time, because the damping force and the internal friction of the shock absorber of the forward landing gear are usually too weak to dampen these oscillations rapidly.
The phenomena described hereinabove have the following disadvantages:
First of all, the maximum loads and stresses applied to the forward landing gear into the structure may be substantial and damaging in terms of the structural mass of the aircraft;
Secondly, the oscillations of the loads and of the stresses at high levels are capable of causing substantial fatigue of the structure and the forward landing gear, as well as internal wear on the landing gear due to the effect of the friction; and
Last, the pitch oscillations can cause discomfort among the crew and passengers.